Liquid Crystal Electro-Optic Device

ABSTRACT

In a horizontal electric field drive type liquid crystal electro-optic device, a gate electrode, a source electrode, a drain electrode, a semiconductor film and a common electrode are formed on a glass substrate and a liquid crystal material is driven by controlling the strength of an electric field substantially parallel to the glass substrate. The electrodes and the semiconductor film are made curved, for example semi-circular or semi-elliptical, in sectional profile. These curved sectional profiles can be formed by suitably selecting and combining various patterning and etching methods.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/272,284, filed Oct. 13, 2011, now allowed, which is a continuation ofU.S. application Ser. No. 12/710,389, filed Feb. 23, 2010, now U.S. Pat.No. 8,040,450, which is a continuation of U.S. application Ser. No.10/807,273, filed Mar. 24, 2004, now U.S. Pat. No. 7,692,749, which is acontinuation of U.S. application Ser. No. 10/305,226, filed Nov. 25,2002, now U.S. Pat. No. 6,914,655, which is a continuation of U.S.application Ser. No. 09/239,066, filed Jan. 25, 1999, now U.S. Pat. No.6,498,634, which is a continuation of U.S. application Ser. No.08/770,703, filed Dec. 19, 1996, now U.S. Pat. No. 5,892,562, whichclaims the benefit of a foreign priority application filed in Japan asSerial No. 7-349670 on Dec. 20, 1995, all of which are incorporated byreference.

BACKGROUND OF THE INVENTION

This invention relates to a liquid crystal electro-optic device havinggood electrical characteristics and angle of visibility characteristicswith which a uniform display can be obtained over an entire screen.

As a method of widening the angle of visibility of a liquid crystalelectro-optic device, a method wherein the direction of an electricfield impressed on a liquid crystal is made substantially parallel tothe surface of a substrate (hereinafter referred to as the super TFTmethod) is disclosed for example in Japanese Unexamined PatentPublication No. H.6-160878. In this case, an electric field is inducedbetween a source electrode and a common electrode formed on onesubstrate, and the liquid crystal molecules are oriented in thedirection of this electric field. Also, in Japanese Unexamined PatentPublication No. H.6-214244, the electric field impressed on the liquidcrystal is made uniform by making the height of the electrodes the sameas the cell thickness.

In this kind of liquid crystal electro-optic device, because switchingis carried out with the long axes of the liquid crystal molecules keptparallel with the substrate, there is no change with angle of visibilityin the optical characteristics of the liquid crystal. Consequently,there is less light leakage and contrast reduction and the likeresulting from angle of visibility than with conventional TN and STNmethods.

However, electrodes of the super TFT method conventionally used havebeen of a trapezoidal or rectangular structure, and the electric fieldsproduced by these electrodes have been noncontinuous at vertices of thetrapezoid or rectangle. Consequently, the electric field impressed onthe liquid crystal has changed at certain points. That is, the electricfield (electric flux density) has changed suddenly at the vertices ofthe trapezoid or rectangle. Consequently, switching of the liquidcrystal by the electric field has not been carried out evenly in thecell, and a phenomenon of the time taken for the electric field tochange from OFF to ON or from ON to OFF (these are respectively calledthe rise time and the fall time) varying within the cell has appeared.

This is a shortcoming which appears particularly markedly in the superTFT method, wherein a horizontal electric field is used to carry outliquid crystal driving.

The above-mentioned electric field noncontinuity will be explained withreference to FIG. 1. Here, for simplicity, the state of lines ofelectric force around the electrodes when a voltage is impressed acrossa pair of parallel electrodes (101, 102) each of a rectangularcross-section of height ‘a’ and width c formed with a spacing 2 bbetween the electrodes on an insulating substrate (103) will bedescribed. (For lines of electric form formed by electric changes,please refer to works on electromagnetism, for example‘Electromagnetism’, Kazukiyo Nagata, published by Asakura, or ‘DetailedElectromagnetic Practice’, Goto and Yamazaki, Kyoritsu publishing.)Here, a direction parallel with the substrate and perpendicular to (theheight direction of) the electrodes will b e made an x-axis and adirection perpendicular to the surface of the substrate will be made ay-axis. An origin will be so defined that the electrode surfacesparallel with the substrate are at y=0.

(1) In the region y<0 (−b≦x≦b), i.e. the region between the electrodes:

Because electric charge can be regarded as being distributed evenly overthe electrode surfaces (104, 105), the lines of electric force (106)here are perpendicular to the electrodes (and parallel with thesubstrate).

(2) In the region y>0, i.e. the region above the electrodes:

Here, for the sake of simplicity, the state of the lines of electricforce in the xy plane will be investigated.

Electric charge can be regarded as being distributed evenly over theelectrode surfaces (107, 108).

For any point in the region y>0, the distance from the origin will bewritten r and the angle made by r and the x-axis will be written θ.Also, expressing z as a point in a complex plane using x, y and r, θ,the following relationship holds:

z=x+iy =rexp(iθ)

Here, to simplify the analysis, a value w will be defined as follows:

w=A log z

(A is a constant of proportionality). If the real and imaginary parts ofw are written u and v, then:

w=u+iv=A log z

and

u+iv=A log {rexp(iθ)}=A log r+iAθ

is obtained. Therefore,

u=A log r, v=Aθ

Therefore, the set of curves expressed u=constant in the w plane are theset of curves r=constant in the xy plane, i.e. the set of concentriccircles about the origin.

This result is illustrated in FIG. 1, from which it can be seen that theelectric field distributions of the electrode side surfaces and theelectrode top surfaces are different.

Here, as an example, the electric field between electrodes whosecross-sections are rectangular was shown, but the situation is the samebetween electrodes whose cross-sections are trapezoidal also This isbecause since electric fields are formed perpendicular to the electrodesurfaces the electric field of the taper parts and the electric field ofthe parts parallel with the substrate are noncontinuous at the electrodevertices.

This kind of noncontinuity of the electric field at the electrodevertices is a problem which cannot be ignored when making very smallpixels. This is because when as a result of the adoption of very smallpixels the number of electrodes increases and the interelectrodedistance becomes small the noncontinuous electric field distributes at ahigh density.

As another method of solving the above-mentioned problem, an inventionwherein in order to impress an electric field on the liquid crystalevenly in the cell thickness direction the height of the electrodes ismade the same as the thickness of the cell has been proposed, inJapanese Unexamined Patent Publication No. H.6-214244. However, inmaking extremely tall electrodes, the following technologicaldifficulties arise.

Firstly, when the height of an electrode is made as great as the cellthickness, a large difference in the horizontal direction electrodethickness tends to arise between the top and the base of the electrode.In the super TFT method, wherein the liquid crystal is driven with ahorizontal electric field, a difference in the electrode thicknessconstitutes a difference in the interelectrode distance. Consequently,because the electric field strength in the cell thickness directionvaries within the same pixel, driving the liquid crystal becomesdifficult.

Secondly, when the electrodes are extremely tall, the coverage of layersformed on top of the electrodes is poor and line breakage tends tooccur.

Thirdly, in making very small pixels, with extremely tall electrodes itis difficult to make the horizontal direction film thickness thin andobtain a large taper angle.

Consequently, in making very small pixels, to solve the above-mentionedproblems, an electrode structure which can be made by a simple methodand which also does not produce a noncontinuous electric field has beenbeing sought.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a liquid crystalelectro-optic device which has an electrode structure such thatnoncontinuity of the electric field strength around each pixel electrodeis minimized and the display characteristics of the device are therebyimproved and which can be made by a simple method.

To achieve this object and other objects, the invention provides aliquid crystal electro-optic device comprising a pair of substrates ofwhich at least one is transparent, electrodes formed on at least one ofthe substrates, a liquid crystal layer held between the substrates, andelectric field impressing means for impressing an electric field on theliquid crystal layer by way of the electrodes, wherein at least one ofthe electrodes has a curved sectional profile.

The invention also provides a liquid crystal electro-optic devicecomprising a pair of substrates of which at least one is transparent,electrodes formed on at least one of the substrates, a liquid crystallayer held between the substrates, and electric field impressing meansfor impressing an electric field on the liquid crystal layer by way ofthe electrodes, wherein at least one of the electrodes has asemi-circular or semi-elliptical sectional profile.

The invention also provides a liquid crystal electro-optic devicecomprising a pair of substrates of which at least one is transparent,electrodes formed on at least one of the substrates including anelectrode for liquid crystal driving and a common electrode having partsformed in parallel on the same substrate, a liquid crystal layer heldbetween the substrates, and electric field impressing means forimpressing an electric field on the liquid crystal layer by way of theelectrodes, wherein at least one of the electrodes has a curvedsectional profile.

The invention also provides a liquid crystal electro-optic devicecomprising a pair of substrates of which at least one is transparent,electrodes formed on at least one of the substrates including anelectrode for liquid crystal driving and a common electrode having partsformed in parallel on the same substrate, a liquid crystal layer heldbetween the substrates, and electric field impressing means forimpressing an electric field on the liquid crystal layer by way of theelectrodes, wherein at least one of the electrodes has a semi-circularor semi-elliptical sectional profile.

The invention also provides a liquid crystal electro-optic devicecomprising a pair of substrates of which at least one is transparent,electrodes formed on at least one of the substrates including anelectrode for liquid crystal driving and a common electrode having partsformed in parallel on the same substrate, a liquid crystal layer heldbetween the substrates, and electric field impressing means forimpressing an electric field on the liquid crystal layer by way of theelectrodes, wherein a nonlinear device is connected to at least one ofthe electrodes and at least one of the electrodes has a curved sectionalprofile.

The invention also provides a liquid crystal device comprising a pair ofsubstrates of which at least one is transparent, electrodes formed on atleast one of the substrates including an electrode for liquid crystaldriving and a common electrode having parts formed in parallel on thesame substrate, a liquid crystal layer held between the substrates, andelectric field impressing means for impressing an electric field on theliquid crystal layer by way of the electrodes, wherein a nonlineardevice is connected to at least one of the electrodes and a peripheraldriving circuit for driving a liquid crystal material is formed on atleast one of the substrates and at least one of the electrodes has acurved sectional profile.

The invention also provides a liquid crystal electro-optic devicecomprising a pair of substrates of which at least one is transparent,electrodes formed on at least one of the substrates including anelectrode for liquid crystal driving and a common electrode having partsformed in parallel on the same substrate, a liquid crystal layer heldbetween the substrates, and electric field impressing means forimpressing an electric field on the liquid crystal layer by way of theelectrodes, wherein a nonlinear device is connected to at least one ofthe electrodes and at least one of the electrodes has a semi-circular orsemi-elliptical sectional profile.

The invention also provides a liquid crystal electro-optic devicecomprising a pair of substrates of which at least one is transparent,electrodes formed on at least one of the substrates including anelectrode for liquid crystal driving and a common electrode having partsformed in parallel on the same substrate, a liquid crystal layer heldbetween the substrates, and electric field impressing means forimpressing an electric field on the liquid crystal layer by way of theelectrodes, wherein a nonlinear device is connected to at least one ofthe electrodes and a periphal driving circuit for driving a liquidcrystal material is formed on at least one of the substrates and atleast one of the electrodes has a semi-circular or semi-ellipticalsectional profile.

The invention also provides a liquid crystal electro-optic devicecomprising a pair of substrates of which at least one is transparent,electrodes formed on at least one of the substrates, a liquid crystallayer held between the substrates, and electric field impressing meansfor impressing an electric field on the liquid crystal layer by way ofthe electrodes, wherein at least one of the electrodes has a curvedsectional profile and the tangential direction of a line of electricforce around the surface of this electrode changes continuously over theentire surface of the electrode.

The invention also provides a liquid crystal electro-optic devicecomprising a pair of substrates of which at least one is transparent,electrodes formed on at least one of the substrates, a liquid crystallayer held between the substrates, and electric field impressing meansfor impressing an electric field on the liquid crystal layer by way ofthe electrodes, wherein at least one of the electrodes has asemi-circular or semi-elliptical sectional profile and the tangentialdirection of a line of electric force around the surface of thiselectrode changes continuously over the entire surface of the electrode.

An example of a construction using the invention disclosed in thisspecification is shown in FIG. 4 and FIG. 5. FIG. 4 is a schematic planview of a pixel part of an active matrix type liquid crystalelectro-optic device wherein nematic liquid crystal is used and thisliquid crystal material is driven with a horizontal electric field anda-Si TFTs are used as the driving devices, and FIG. 5 is a sectionalview on the line A-A′ in FIG. 4.

In the construction shown in FIG. 4 and FIG. 5, 401 denotes first andsecond substrates, 402 a base SiO₂ film, 403 a gate electrode, 404 acommon electrode, 405 a gate insulating film, 406 a-Si, 407 a sourceelectrode, 408 a drain electrode, 409 a protective layer, 411 anorienting film, 412 a polarizing plate and 413 a liquid crystal layer.

The liquid crystal electro-optic device of this invention is one whereina liquid crystal material is operated by controlling the strength of anelectric field (a horizontal electric field) between a drain electrodeand a common electrode formed on a TFT substrate.

For the above-mentioned first and second substrates, a transparentmaterial having a certain degree of strength with respect to outsideforces, for example an inorganic material such as glass or quartz, isused. For the substrate on which the TFTs are formed (hereinafter calledthe TFT substrate), non-alkali glass or quartz glass is used. When alightweight liquid crystal electro-optic device is to be made, a filmhaving little birefringence, for example PES (Poly Ethylene Sulfate) orthe like also can be used.

As the method by which the liquid crystal material is driven, themultiplex method or the active matrix method may be used.

With the multiplex method, all that need be formed on the firstsubstrate are electrodes for display and reference electrodes, but inthe case of the active matrix method, in addition to these a nonlineardevice, for example a thin film transistor (TFT) or a diode, is formedfor each pixel as a switching device.

As the TFT, a transistor in which amorphous silicon orpolysilicon(polycrystalline silicon) is used as an active layer can beused. In the case of the active matrix method, as the construction ofthe driving device, a known construction such as the stagger type or thereverse stagger type can be used. In the case of a transistor whereinpolysilicon is used, it is possible to form a peripheral driving circuitfor driving the liquid crystal material on the substrate on which theTFTs are formed. The peripheral driving circuit can be formed in thesame process as that by which the TFTs are made. This peripheral drivingcircuit is made up of complementary devices wherein n-channel andp-channel transistors are combined.

As the device electrodes, Cr, Al, ITO and Ta can be used. The sectionalprofiles of the electrodes are made smoothly sloping or curved by amethod shown below. A sectional profile forming a smoothly slopingsurface or a curved surface shown in this specification can be made by adry process or a wet process. Examples of dry processes include:

(a) methods wherein anisotropic plasma etching and isotropic plasmaetching are combined; and

(b) methods wherein plasma isotropic etching is carried out using amask.

As a method of category (a) above, a mask is patterned on a n electrodeand anisotropic plasma etching is carried out. The mask is then removed,and resist is coated onto parts not to be isotropically plasma etched.After that, isotropic plasma etching is carried out without a mask onparts to be given a curved sectional profile. In this way, projectingparts are shaved off and it is possible to make an electrode having asmoothly sloping curved sectional profile. After that, the resist isremoved. As a method of category (b) above, it is possible to obtain aneat arcuate sectional profile b y suitably setting a discharge gasvoltage.

In a wet process, on the other hand, as the resist, one whose etchingselection ratio is not much different from that of the electrode beingetched is used. Also, a resist whose taper angle is somewhat small isused. When this is done, the mask and the electrode being etched areetched by wet etching at about the same rate. In this way, it ispossible to make an electrode having a smoothly sloping curved sectionalprofile with rounded vertices.

The above-mentioned methods are just examples of methods for makingelectrodes having smoothly sloping curved sectional profiles, and themethod by which an electrode having a smoothly sloping curved sectionalprofile of the invention is made is not limited to these methods.

If one of the electrode materials mentioned above is used, by forming anoxide film of the metal constituting the electrode material on theelectrode surface by a method such as anodic oxidation after the curvedsectional profile is formed as described above, it is possible to makethis an interlayer insulating film. In this way, it is possible toimprove interelectrode insulation even in cases of constructions whereinadjacent electrodes or electrode patterns overlap.

Also, it is possible to use silicon oxide (SiO₂) or silicon nitride(SiN) as interlayer insulating films and TFT protecting layers.

For the opposing substrate, the same material as that used for thesubstrate on which the TFTs are formed can be used. Also, although it isnot particularly necessary to form any electrodes on the opposingsubstrate, in some cases electrodes 414 may be formed on all or part ofthe opposing substrate as shown in FIG. 8A. As the electrode material inthis case, besides the above-mentioned metals, a material havingtransparency, for example ITO or the like, can be used.

To improve contrast by blocking light from parts not contributing todisplay, a black matrix 415 is formed on the opposing substrate or theTFT substrate or both substrates using a metal such as Cr or a resinmaterial in which a black pigment has been dispersed as shown in FIG.8B. Also, in the case of color display, R (red), G (green), B (blue) orC (cyan), M (magenta), Y (yellow) color filters are formed in positionscorresponding to respective pixels. As the arrangement of the colors ofthe color filters, a stripe arrangement or a delta arrangement or thelike can be used.

After that, an orienting process is carried out on the substrate onwhich the driving devices are formed and on the opposing substrate. Thisorienting process is carried out so that the liquid crystal moleculesare parallel with the substrate and oriented uniaxially. As theorienting process, rubbing treatment wherein the substrate surface orthe surface of an organic resin film of nylon or polyimide or the like(orienting film) (411) formed on the substrate is rubbed in onedirection is effective.

The rubbing direction differs according to the liquid crystal material(413) used, and on the TFT substrate side, in the case of a liquidcrystal material whose dielectric constant anisotropy is positive, therubbing direction is made a direction not parallel to the electricfield, and preferably at 45° to the electric field. In the case of amaterial whose dielectric constant anisotropy is negative, the rubbingdirection is made a direction not orthogonal to the electric field, andpreferably 45° to the electric field. Rubbing of the opposing substrateside is carried out in a direction parallel or oppositely parallel tothe rubbing direction of the TFT substrate.

The pair of substrates thus made are brought face-to-face with eachother with a fixed spacing therebetween to form a liquid crystal cell. Asealing agent (not shown) as an adhesive is formed in a predeterminedpattern on one of the substrates. As the sealing agent, a resin materialhardened thermally or by ultraviolet rays is used. As this resinmaterial, an epoxy or urethane acrylate material can be used. Spacers(not shown) for maintaining the spacing between the two substrates overthe whole cell are distributed on the other substrate. After the sealingagent is hardened, the liquid crystal material is injected into theliquid crystal cell by vacuum injection or the like.

Examples of liquid crystal materials which can be used in this inventioninclude nematic, cholesteric and smectic materials, but using a nematicmaterial is particularly preferable. Also, from among nematic liquidcrystals, one whose dielectric constant anisotropy is positive or onewhose dielectric constant anisotropy is negative is suitably chosenaccording to the driving method. Also, to reduce the influence ofbirefringence, a liquid crystal material whose refractive indexanisotropy is small is preferable.

In a liquid crystal electro-optic device of the invention, to carry outdisplay utilizing the birefringence of the liquid crystal material, apair of polarizing plates (412) are arranged with their optical axesintersecting orthogonally and the liquid crystal cell is sandwichedbetween this pair of polarizing plates. At this time, the orientationdirection of the liquid crystal material is parallel with the opticalaxis of the analyzer, i.e. the polarizing plate nearer the light source.

In a liquid crystal electro-optic device made in this way, theorientation of the liquid crystal material is such that when there is noelectric field the long axis of the liquid crystal material isuniaxially oriented in parallel with the substrate and in parallel withthe rubbing direction. Then, when an electric field is impressed, theliquid crystal molecules in the vicinities of the orienting filmsurfaces, which are subject to a strong orientation restricting force,remain parallel with the rubbing direction while the optical axes of theliquid crystal molecules in the vicinity of the middle of the liquidcrystal layer, which are only subject to a weak orientation restrictingforce, are changed by the electric field. When a liquid crystal materialwhose dielectric constant anisotropy is positive is used, the long axesof the liquid crystal molecules become oriented in parallel with theelectric field direction, and when the dielectric constant anisotropy isnegative the long axes of the liquid crystal molecules become orientedperpendicular to the electric field direction.

Consequently, with respect to light passing through the liquid crystalelectro-optic device, because when there is no electric field theorientation of the liquid crystal material inside the cell is parallelwith the optical axis of the analyzer, incident light cannot passthrough the polarizer and the amount of light passing through at thistime is zero. When an electric field is impressed, on the other hand,the orientation of the optical axis of the liquid crystal materialchanges and consequently incident light becomes elliptically polarizedlight and passes through the polarizer.

In the construction described above, two polarizing plates are used, butif a reflecting plate made of metal or the like is formed on one of thetwo substrates, it is possible to make the liquid crystal electro-opticdevice using only one polarizing plate, and a bright display can berealized. The metallic reflecting plate can also double as for example apixel electrode.

When a liquid crystal electro-optic device is constructed according tothis invention, compared to electrodes having rectangular or trapezoidalsectional profiles which have been used in conventional liquid crystalelectro-optic devices, the electric field around the electrodes iscontinuous. This continuity of the electric field is clear from thestate of lines of electric force around the electrodes when a voltage isimpressed on the electrodes. The state of lines of electric force aroundelectrodes will now be described in detail with reference to FIG. 2.

First, for simplicity, a case wherein point charges q₁, q₂ exist atpoints O₁, O₂ will be considered.

Here, the straight line joining O₁, O₂ will be taken as an x-axis and adirection perpendicular to the x-axis will be made a y-axis. An originwill be defined as the point half-way between the points O₁, O₂.

A line of electric force passing through any point P as shown in FIG. 2will be considered. This is in the plane formed by the point P and thepoints O₁, O₂.

When this line of electric force is rotated about the O₁, O₂ axis, asurface of rotation is obtained, and the electric flux passing throughany cross-section of this surface of rotation should be constant.

The electric flux passing through a vertical section S passing through Pwill now be obtained.

If the angles made by the lines O₁P and O₂P and the O₁O₂ axis arerespectively written θ₁, θ₂, the electric flux φ₁ passing through S dueto q₁ is:

φ₁ =q ₁·2π(1−cos θ₁)/4π

and the electric flux φ₂ passing through S due to q₂ is:

φ₂ =q ₂·2π(1−cos θ₂)/4π

and therefore the total electric flux φ passing through S is given by:

φ=1/2{(q ₁ +q ₂)−(q ₁ cos θ₂)}

Therefore, on one line of electric force,

q ₁ cos θ₁ =q ₂ cos θ₂=constant

If

q ₁ =−q ₂,

on the line of electric force there is the relationship:

cos θ₁−cos θ₂=constant

Lines of electric force (110) and equipotential surfaces (111) formed bythis pair of point charges are shown in FIG. 3.

The distribution of these lines of electric force is the same even ifcharges the same size as the above-mentioned point charges aredistributed on a conducting surface of radius ‘a’. Also, in the regiony≧0, it can be approximated to an electric field created by twosemi-circular electrodes. Therefore, if the electrode sectional profilesare semi-circular, the distribution of the lines of electric force iscontinuous with respect to the cell thickness direction.

In the above description, an example wherein the entire sectionalprofile of the electrode has a circular curvature was shown, but theinvention is not limited to this and the same effects can be obtainedwith an electrode having an elliptical curvature. Also, the sectionalprofile does not have to form a regular semi-circle, and the sameeffects can be obtained with a sectional profile forming an arc. Theelectrode edge section may have a curved surface of a circular arc shapeor the like. An electrode having a sectional profile having a polygonalshape with gentle boundary changes may of course also be used.

Also, films formed on thin films such as electrodes having smoothlysloping curved sectioned profiles have good coverage, because ofroundness of the thin films. Therefore, there is also the effect ofpreventing mixing in of impurities and line breakage caused by poorcoverage.

The technique of this invention of making the sectional profile of anelectrode curved or smoothly sloping can of course be applied not onlyto the above-mentioned a-Si type TFTs but also to poly-Si type TFTs.

In particular, when poly-Si is used for the active layer of a TFT,because the carrier mobility of the active layer is larger than whena-Si is used for the active layer and consequently the samecharacteristics as an a-Si transistor can be obtained with a smallerdevice region, devices can be made small and therefore a high percentageaperture can be realized. Also, in impressing a horizontal electricfield, a higher response speed can be realized when poly-Si, having alarge carrier mobility, is used for the TFT active layer. Furthermore,when poly-Si is used, it is possible to also form a peripheral drivingcircuit for driving the liquid crystal material on the substrate andthis contributes to reduction of the number of steps required tomanufacture the device, improvement of yield and reduction of the priceof the device.

The invention has been discussed above with reference to a liquidcrystal electro-optic device of a type wherein a horizontal electricfield is impressed on a liquid crystal material; however, the inventionis not limited to this and can also be used in a liquid crystalelectro-optic device of a type wherein a vertical electric field isimpressed on liquid crystal material, for example a conventional TN typeor the like, whereby disturbances in the electric field at the ends canbe reduced and it is possible to make an electro-optic device havinggood coverage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing lines of electric force of when anelectric field is impressed across electrodes in a conventional liquidcrystal electro-optic device;

FIG. 2 is a simplified view of lines of electric force and equipotentialsurfaces formed by two point charges;

FIG. 3 is a view showing lines of electric force and equipotentialsurfaces around a pair of electrodes having a curved sectional profile;

FIG. 4 is a schematic plan view of a pixel region of a liquid crystalelectro-optic device of a first preferred embodiment of the invention;

FIG. 5 is a schematic sectional view on the line A-A′ in FIG. 4;

FIG. 6 is a schematic plan view of a pixel region of a liquid crystalelectro-optic device of a second preferred embodiment of the invention;and

FIGS. 7(A) to 7(E) are schematic sectional views on the line B-B′-B″ inFIG. 6 showing the device at different stages in the process of itsmanufacture.

FIGS. 8A and 8B are schematic sectional views of the opposing substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the invention will now be described.

First Preferred Embodiment

A silicon oxide film of thickness 1000 to 3000 Å was formed as a baseoxide film (402) on a Corning #7059 insulating substrate (401). As themethod of forming this silicon oxide film, sputtering in an oxygenatmosphere or plasma CVD can be used. A film of Cr was then formed onthis to a thickness of 1000 to 5000 Å and patterned. After that,isotropic plasma etching was carried out using resist as a mask. At thistime, the discharge gas voltage was suitably set to give the electrodescurved surfaces. In this way, a gate electrode (403) and a commonelectrode (404) were formed.

A gate insulating film (405) consisting of silicon dioxide (SiO₂) wasthen formed so as to cover these electrodes. This film may alternativelyconsist of silicon nitride (SiN). An amorphous silicon film (406) wasthen formed on the gate insulating film above the gate electrode. Then,a source electrode (407) and a drain electrode (408) consisting of Alwere formed so as to overlap with parts of the pattern of the amorphoussilicon film. At this time, isotropic plasma etching was carried outusing resist as a mask, and the discharge gas voltage was suitably setto give the electrodes curved surfaces. A silicon oxide insulating film(409) was then formed as a TFT protecting film. This insulating film mayalternatively be an SiN film.

Also, on the opposing substrate or on the TFT substrate or on bothsubstrates, to improve contrast, a black matrix consisting of a metalsuch as Cr or a resin in which a black pigment is dispersed was formedto block light from parts not participating in display.

After that, an orienting film (411) consisting of polyimide was formedon the substrate on which the TFT was formed and on the opposingsubstrate. As the orienting film, polyimide was formed by a known spincoating or dipping method. The orienting film surfaces were then rubbed.

The rubbing direction differs according to the liquid crystal materialused, and on the TFT substrate side, in the case of a liquid crystalmaterial whose dielectric constant anisotropy is positive, the rubbingdirection is made a direction not parallel to the electric field, andpreferably at 45° to the electric field. In the case of a material whosedielectric constant anisotropy is negative, the rubbing direction ismade a direction not orthogonal to the electric field, and preferably at45° to the electric field. Rubbing of the opposing substrate side iscarried out in a direction parallel or oppositely parallel to therubbing direction of the TFT substrate.

The TFT substrate and the opposing substrate thus formed were broughtface-to-face to form a liquid crystal panel. Spherical spacers ofdiameter 3 μm were interposed between the two substrates to obtain auniform substrate spacing over the entire panel. The two substrates werethen sealed with an epoxy adhesive to fix them together. The pattern ofthe sealing was made such that it surrounded the pixel region and aperipheral circuit region. After that, the pair of substrates were cutto a predetermined shape and a liquid crystal material was then injectedbetween them.

Two polarizing plates (412) were then affixed to the outer sides of thesubstrates. The polarizing plates were so oriented that their opticalaxes intersected orthogonally and the optical axis of one of thepolarizing plates was made parallel with the rubbing direction.

When the optical characteristics of this liquid crystal electro-opticdevice were measured, good display having less dispersion in risecharacteristics than a liquid crystal display having conventionalelectrode shapes was obtained.

Second Preferred Embodiment

The liquid crystal electro-optic device of this preferred embodiment isa monolithic active matrix circuit wherein a peripheral driving circuitis also formed on the substrate. A process for making the device will bedescribed using FIG. 6 and FIGS. 7(A) to 7(E). FIG. 6 is a schematicplan view of a pixel of this preferred embodiment. FIGS. 7(A) to 7(E)are sectional views on the line B-B′-B″ in FIG. 6, a process formanufacturing TFTs of a driving circuit being shown on the left side anda process for manufacturing a TFT of an active matrix circuit beingshown on the right side. These processes are low temperature polysiliconprocesses.

First, a base silicon oxide film (402) was formed on a Corning #1737first insulating substrate (601). This silicon oxide film may be formedby the same method as that shown in the first preferred embodiment.

After that, an amorphous silicon film was formed to 300 to 1500 Å, andpreferably 500 to 1000 Å, by plasma CVD or LPCVD. Thermal annealing wasthen carried out at a temperature of over 500° C., and preferably 500 to600° C., whereby the silicon film was crystallized or its crystallinitywas raised. After this crystallization by thermal annealing, light(laser or the like) annealing may be carried out to further increasecrystallinity. Also, as shown in Japanese Unexamined Patent PublicationsNos. H.6-244103 and H.6-244104, at the time of crystallization bythermal annealing, an element such as nickel or the like which promotesthe crystallization of silicon (a catalyst element) may be added.

The silicon film was then etched to form island-shaped active layers(602) (for a P-channel type TFT) and (603) (for an N-channel type TFT)of the TFTs of the driving circuit and an active layer (604) of the TFTof the matrix circuit (a pixel TFT). Also, a silicon oxide gateinsulating film (605) of thickness 500 to 2000 Å was formed bysputtering in an oxygen atmosphere. As the method of forming the gateinsulating film, plasma CVD may alternatively b e used. When forming asilicon oxide film by plasma CVD, as the raw material gas, usingnitrogen monoxide (N₂O) or oxygen (O₂) and monosilane (SiH₄) waspreferable.

After that, aluminum of thickness 2000 to 6000 Å was formed bysputtering over the entire surface of the substrate. Here, to preventhillocks forming in a subsequent heating process, aluminum containingsilicon or scandium or palladium or the like may be used. Then, gateelectrodes (606, 607, 608) and a common electrode (609) were formed byisotropic plasma etching (FIG. 7(A)). At this time, the discharge gasvoltage was suitably set to give the electrodes curved surfaces. Afterthat, by ion doping, utilizing self-alignment with the gate electrodes(606, 607, 608) as masks, with phosphine (PH₃) as the doping gas,phosphorus was doped into all the island-shaped active layers. The doseamount was 1×10¹² to 5×10¹³ atoms/cm².

As a result, weak N-type regions (610, 611, 612) were formed. (FIG.7(B)).

Next, a photoresist mask (613) covering the P-channel type active layer(602) and a photoresist mask (614) covering the active layer (604) ofthe pixel TFT as far as 3 μm from the ends of the gate electrode (608)in parallel with the gate electrode were formed.

Then, phosphorus was again injected by ion doping with phosphine as thedoping gas. The dose amount was 1×10¹⁴ to 5×10¹⁵ atoms/cm^(2.) As aresult of this, strong N-type regions (source and drain) (615, 616) wereformed. The region (617) covered by the photoresist mask (614) on thepixel TFT remained weak N-type because no phosphorus was injected intoit in this doping. (FIG. 7(C)).

Next, the N-channel type active layers (603, 604) were covered with aphotoresist mask (618), and boron was injected into the island-shapedregion (602) by ion doping with diborane (B₂H₆) as the doping gas. Thedose amount was 5×10¹⁴ to 8×10¹⁵ atoms/cm². In this doping, because thedose amount of boron is greater than the dose amount of phosphorus inFIG. 7(C), the previously formed weak N-type region (610) inverts into astrong P-type region (619).

By the doping described above, strong N-type regions (source/drain)(615, 616), a strong P-type region (source/drain) (619) and a weakN-type region (low concentration impurity region) (617) were formed.(FIG. 7(D))

After that, by carrying out thermal annealing at 450 to 850° C. for 0.5to 3 hours, damage caused by the doping was repaired, the dopedimpurities were activated and the crystallinity of the silicon wasrestored. After that, a silicon oxide film was formed over the entiresurface as an interlayer insulating film (620) to a thickness of 3000 to6000 Å by plasma CVD. This may alternatively be a silicon nitride filmor a multiple layer film comprising a silicon oxide film and a siliconnitride film. The interlayer insulating film (620) was etched by wetetching or dry etching to form contact holes above the source and drainregions.

Then, an aluminum film or a multiple layer film comprising titanium andaluminum of thickness 2000 to 6000 Å was formed by sputtering. This wasthen isotropically plasma etched using resist as a mask. At this time,the discharge gas voltage was suitably set to give the electrodes curvedsurfaces, and electrodes/interconnections (621, 622, 623) of theperipheral circuit and electrodes/interconnections (624, 625) of thepixel TFT were formed.

Also, a silicon nitride film (626) of thickness 1000 to 3000 Å wasformed as an interlayer film by plasma CVD. (FIG. 7(E)

Thereafter, by the same method as in the first preferred embodiment, aliquid crystal cell was made. Here, the pattern of the seal was madesuch that it enclosed the pixel region and the peripheral drivingcircuit region. Also, after that, polarizing plates were affixed to thepair of substrates as in the first preferred embodiment to complete theliquid crystal electro-optic device.

When the optical characteristics of this liquid crystal electro-opticdevice were measured, good display having less dispersion in risecharacteristics than a liquid crystal display having conventionalelectrode shapes was obtained.

With the construction of this preferred embodiment, because the drivingcircuit is made on the same substrate as the pixel TFT, there is themerit that the manufacturing cost is low.

As described above, with this invention it is possible to obtain with asimple manufacturing process a liquid crystal electro-optic device whoseliquid crystal rise characteristics are better than those of aconventional horizontal electric field drive type liquid crystalelectro-optic device. The invention also allows pixel size reduction.

What is claimed is:
 1. A display device comprising: a transistor over afirst substrate, the transistor including a semiconductor layer and agate electrode; a drain electrode electrically connected to thesemiconductor layer; a common electrode partly overlapped with the drainelectrode; a liquid crystal over the drain electrode and the commonelectrode; and a second substrate over the liquid crystal, wherein anelectric field is applied to the liquid crystal substantially inparallel with a surface of the first substrate by the common electrodeand the drain electrode, and wherein a transparent conductive materialis provided over the second substrate.
 2. The display device accordingto claim 1, wherein each of the drain electrode and the common electrodehas a curved cross section.
 3. The display device according to claim 1,wherein the gate electrode and the common electrode are formed over andin contact with a same surface of a base silicon oxide film.
 4. Thedisplay device according to claim 1, wherein the semiconductor layerincludes amorphous silicon.
 5. The display device according to claim 1,wherein the transparent conductive material functions as an electrode.6. A display device comprising: a transistor over a first substrate, thetransistor including a semiconductor layer and a gate electrode; a drainelectrode electrically connected to the semiconductor layer; a commonelectrode partly overlapped with the drain electrode; a liquid crystalover the drain electrode and the common electrode; and a secondsubstrate over the liquid crystal, wherein a silicon nitride film isinterposed between the common electrode and the drain electrode, whereinan electric field is applied to the liquid crystal substantially inparallel with a surface of the first substrate by the common electrodeand the drain electrode, and wherein a transparent conductive materialis provided over the second substrate.
 7. The display device accordingto claim 6, wherein each of the drain electrode and the common electrodehas a curved cross section.
 8. The display device according to claim 6,wherein the gate electrode and the common electrode are formed over andin contact with a same surface a base silicon oxide film.
 9. The displaydevice according to claim 6, wherein the semiconductor layer includesamorphous silicon.
 10. The display device according to claim 6, whereinthe transparent conductive material functions as an electrode.